Technical Field
The invention relates to substituted pyrazolopyrimidines that are positive allosteric modulators of the γ-aminobutyric acid receptor (e.g., GABA-B PAM), useful in treating diseases and conditions mediated and modulated by the γ-aminobutyric acid receptor B. Additionally, the invention relates to compositions containing compounds of the invention and processes of their preparation.
Description of Related Technology
The inhibitory neurotransmitter, γ-aminobutyric acid (GABA) exerts its actions through three distinct receptors—the ionotropic GABA-A and GABA-C receptors, and the metabotropic GABA-B receptor. The GABA-B receptor is a member of the class C family of GPCRs. The GABA-B receptor is an obligate heterodimer composed of a GABA-B1 and a GABA-B2 subunit (Bettler, B., et al. Physiol Rev 2004; 84: 835-867). Notably, heterodimerization of the B1 and B2 subunits is required for proper GABA-B receptor expression and function (Pin, J. P., et al. Biochem Pharmacol 2004; 68: 1565-1572). Agonist binding to the B1 subunit of the GABA-B heterodimer results in transactivation of the B2 subunit and subsequent stimulation of Gi/o proteins. This, in turn, activates K+ currents, inhibits Ca2+ currents, and decreases cAMP via negative regulation of adenylyl cyclase.
GABA-B receptor subunits are found both pre- and post-synaptically throughout the CNS and the periphery, with highest expression in the thalamus, cortex, cerebellum and dorsal horn (Fritschy, J. M., et al. Eur J Neurosci 1999; 11: 761-768). Functional receptor expression appears to be limited by the presence of the GABA-B2 subunit, which is often detected at lower levels than the B1 subunits (Bowery, N. G. Adv Pharmacol 2010; 58: 1-182). Therapeutically, the beneficial effects of GABA-B receptor stimulation include muscle relaxation, substance abuse treatment (especially in alcohol dependence), antinociception, spasticity, fragile X syndrome, Down's syndrome, autism, retinal ganglion cell degeneration, gastro-esophageal reflux disease (GERD), smoking cessation, addiction of narcotic agents, emesis, cough, overactive bladder, anxiety, migraine or tinnitus. The in vivo consequence of GABA-B activation has been confirmed experimentally with knockout mouse models, which are hyperalgesic, and clinically with the GABA-B orthosteric agonist, baclofen (Slonimski, M., et al. Reg Anesth Pain Med 2004; 29: 269-276. Bowery, N. G., et al. Pharmacol Rev 2002; 54: 247-264). Unfortunately, the utility of baclofen is limited by drug tolerance and severe side effects, including hypothermia, seizures, sedation and cognitive deficits. Baclofen has poor brain penetrance and requires high doses for engagement of CNS GABA-B receptors, resulting in elevated plasma concentrations. Peripheral GABA-B receptors on smooth and skeletal muscle are activated by these high plasma concentrations and appear to mediate a subset of baclofen side effects, including hypothermia and seizures that are hypothesized to arise from erratic muscle contractions. Drug tolerance requiring dose escalation has been reported three to seven days after initiation of baclofen and most likely arises from receptor desensitization (Sands, S. A., et al. J Pharmacol Exp Ther 2003; 305: 191-196). This reduction in GABA-B receptor signaling mimics the genotype of GABA-B knockout mice, which also exhibit severe cognitive and learning deficits (Schuler, V., et al. Neuron 2001; 31: 47-58). Tolerance and receptor desensitization following repeated baclofen administration may therefore underlie the cognitive deficits and learning impairments associated with baclofen treatment. Finally, abrupt discontinuation of intrathecal baclofen results in severe withdrawal symptoms, including seizures (Ross, J. C., et al. Neurocrit Care 2011; 14: 103-108). This indicates an underlying change in the physiological balance of GABA neurotransmitter and GABA-B receptor after continued exogenous agonist stimulation. Similarly, the GABA-B knockout mouse exhibits epileptiform seizures, further underscoring the importance of maintaining the normal, physiological GABA-B tone within the CNS. While the GABA-B receptor remains a valid drug target, the side effects and tolerance associated with baclofen emphasize the need to pursue alternatives to classic orthosteric activation of the receptor.
To exploit the beneficial aspects of GABA-B stimulation, the disclosure describes positive allosteric receptor ligands for modulation of the GABA-B receptor. Positive allosteric modulators alter the receptor conformation and enhance the activity of the endogenous orthosteric agonist, either by increasing the affinity or the efficacy of the orthosteric ligand at the receptor (Wang, L., et al. J Pharmacol Exp Ther 2009; 331: 340-348). Because allosteric modulators rely on local levels of endogenous ligand and have little or no activity of their own, they are thought to represent a safer and more subtle means of receptor regulation. The hypothesis is that GABA-B receptor allosteric modulators, and possibly allosteric agonists, will be effective therapeutic agents while minimizing the side effects caused by agonist activation of the orthosteric GABA-B site. In addition to pain indications (Anghinah, R., et al. Muscle Nerve 1994; 17(8): 958-959. Fromm G. H., et al. Ann. Neuro. 1984; 15: 240-244.), GABA-B modulators could also be used in the treatment of depression, spasticity (Bowery, N. G. Curr Opin Pharmacol 2006; 6; 37-433. Froestl, W. Expert Opin Ther Pat 2010; 20: 1007-1017. Ong, J., et al. CNS Drug Rev 2005; 11: 317-334.)), fragile X syndrome (Lozano, R., et al. Neuropsychiatric Disease and Treatment 2014; 10: 1769-1779), Down's syndrome (Kleschevnikov, A. M., et al. Journal of Neuroscience 2012; 32(27): 9217-9227), autism (Oblak, A. L., et al. Journal of Neurochemistry 2010; 114(5): 1414-23), retinal ganglion cell degeneration (Hirano, A. A., et al. Journal of Comparative Neurology 2005; 488(1): 70-81), gastro-esophageal reflux disease (GERD) (Lacy, B. E., et al. Drugs of the Future 2010; 35(12): 987-992. Boeckxstaens, C. E. et al. Current Opinion in Pharmacology 2008; 8(6): 685-689. Lehmann, A., et al. Advances in Pharmacology 2010; 58: 287-313) smoking cessation (Vlachou, S., et al. Psychopharmacology 2011; 215(1): 117-128), addiction of narcotic agents (Spano, M. S., et al. Neuropharmacology 2007; 52(7): 1555-62), emesis (Sanger, G. J., et al. Autonomic Neuroscience 2006; 129(1-2): 3-16), cough (Bolser, D. C., et al. British Journal of Pharmacology 1993; 110(1): 491-495), overactive bladder (Taylor, M. C., et al. British J. Urology 1979; 51: 504-505), anxiety (Krupitsky, E. M., et al. Drug and Alcohol Dependence 1993; 33: 157 163. Cryan, J. F., et al. J Pharmacol Exp Ther 2004; 310: 952-963. Mombereau, C., et al. Neuropsychopharmacology 2004; 29: 1050-1062), migraine (Hering-Hanit, R., Cephalalgia 1999; 19(6): 589-91. Hering-Hanit, R., et al. Headache 2000; 40(1): 48-51.), and tinnitus (Smith, P. F., et al. Frontiers in Neurology 2012; 3: 34). Positive allosteric modulators (PAMs) bind to functionally and topographically distinct allosteric sites on the receptor and act at a distance from the orthosteric site to enhance the efficacy of the endogenous ligand. A single receptor may possess multiple, discrete allosteric sites, each with a unique subset of ligands. Pure PAMs are devoid of activity on their own they will only enhance the potency and/or efficacy of the endogenous agonist thus their pharmacological profile is spatially and temporally controlled by the normal physiological interaction between the endogenous ligand and its receptor. This highlights a critical difference between PAMs and orthosteric agonists PAMs avoid the maximum on/off at all receptors that occurs with classic orthosteric agonist stimulation. Because PAMs rely on endogenous agonist concentrations for activity, they promote fine-tuning of the GABA signal in a physiologically-relevant manner. Importantly, GABA-B PAMs do not cause receptor desensitization (Gjoni, T., et al. Neuropharmacology 2008; 55: 1293-1299), so the clinical tolerance and side effects related to receptor desensitization that are observed with baclofen are unlikely to occur. Finally, the majority of GABA-B PAMs tested in the literature (Brusberg, M., et al. Neuropharmacology 2009; 56: 362-367. Froestl, W. Expert Opin Ther Pat 2010; 20: 1007-1017. Koek, W., et al. J Pharmacol Exp Ther 2010; 335: 163-17. Pin, J. P., et al. Curr Neuropharmacol 2007; 5, 195-201.) show greatly enhanced brain penetrance as compared to baclofen and excellent efficacies in preliminary studies with minimum or no side effects. These collective data emphasize the need for alternative therapeutics at the GABA-B receptor, and highlight the unique ability of PAMs to stimulate the receptor without baclofen-like side effects.